This invention relates to a circuit for the line synchronisation in a picture display device, comprising a line oscillator, a phase discriminator having an input for receiving a signal generated by the oscillator, an input for receiving an incoming line synchronising signal and an output, a loop filter for smoothing the signal at the output of the phase discriminator and for applying the obtained control signal to a control input of the line oscillator for controlling the frequency and/or the phase of the oscillator signal.
A circuit of this type is generally known for use in picture display devices, for example, television receivers. If for some reason or other a variation of the frequency and/or the phase of the oscillator signal with respect to the incoming line synchronising signal occurs in this known circuit, the control loop constituted by means of the phase discriminator, the loop filter and the line oscillator ensures that the synchronism between the two signals is substantially restored in the course of time.
However, if the incoming video signal, from which the synchronising signal applied to the line synchronising circuit is derived, originates from a video recorder, a phase jump may occur in the series of the incoming line synchronising pulses at the end of a field. It is a requirement that this jump, expressed in time, covers at most 16 .mu.s, i.e. a quarter of a line period. Known circuits do not completely correct the perturbation of the synchronising circuit caused at the end of the field blanking interval, that is to say, before the part of the video signal to be displayed which is visible on a display screen begins, resulting in the phase of the line oscillator signal being incorrect at the beginning of the field for a number of line periods so that the picture is displayed on the screen in a distorted manner such that vertical lines at the top of the screen are crooked.
This error can be prevented by using known measures. For example, the picture display device may have a facility for an accelerated lock-in during the field blanking interval by switching the line phase control loop during this interval upon reception of video recorder signals, more specifically in such a manner that the loop gain of the loop in this interval acquires a higher value, or the time constant of the loop filter in the form of a low-pass filter is reduced. However, these and similar measures have the drawback that the behaviour of the control loop in the case of small phase errors is not optimum and that the circuit will be too sensitive to noise and interference.